In antennas, size dictates bandwidth because field expansion occurs at a finite rate, given by the speed of light. This gives rise to the well known size-bandwidth limitation known as Chu's limit (CHU, L. J.: “Physical Limitations In Omnidireactional Antennas”, J Appl. Phys, 1948, 19, pp. 1163-1175).
Newer designs and manufacturing techniques have driven electronic components to small dimensions and miniaturized many communication devices and systems. Unfortunately, antennas have not been reduced in size at a comparative level and often are one of the larger components used in a smaller communications device. To reduce antenna size, relative to free space wavelength, loading is typically used. Loading may take various forms, including, circuit loading and material loading.
In dielectric material loading, an antenna may be placed in proximity with dielectric compounds. For example, a thin wire dipole may be cast into a cake of paraffin. Or, a dielectric puck may be placed along a slot antenna, such as a planar inverted F (PIFA) antenna.
In magnetic material loading, an antenna is used in proximity with permeable magnetic compounds. An example is the “ferrite loopstick” antenna; commonly used for medium frequency (MF) broadcast reception. The ferrite loopstick usually includes multiple wire turns on a slender ferrite rod, in which permeability greatly exceeds permittivity (μr>>∈r). In this loading, dielectric loading effects are nominal, and controlled wave expansion is not an objective. Specifically, ferrite is configured only inside the winding, where it does not interact directly with the radio waves.
In resistive or dissipative material loading, antennas are configured with lossy materials. For example, an antenna may placed inside an absorber, such as graphite impregnated foam. In resistive loading, radiation efficiency is traded for an increase in VSWR bandwidth. Unfortunately, resistive loading decreases radiation bandwidth and gain.
Prior art material loadings therefore, dielectric, or magnetic, provide antenna miniaturization but at a decrease in instantaneous radiation bandwidth. A broadband approach of antenna loading and miniaturization is needed for wideband communications.
One definition of electrically small involves a spherical envelope of d<λ/2π, where d is the diameter of the sphere, and λ is the free space wavelength. An electrically small antenna fits inside this spherical envelope, commonly referred to as a radian sphere.
Radomes can be hollow spherical shells that enclose antennas. They are routinely used for weather protection, and they can provide loading to the antenna. They can be bandwidth limiting, unless they are electrically thin in structure. Thick, strong radomes, commonly operate near even multiples of ½ wavelength thickness, and are bandwidth limited to about ½ octave or less. Thin radomes have more bandwidth, but may be mechanically weak.
The canonical antennas are the line and the circle, which are known in the art as the dipole antenna and the loop antenna. In the dipole antenna, charge is separated, while in the loop, charge is conveyed. Both have been attributed to Hertz. While the line and circle antenna are linearly polarized, when configured together they can provide circular polarization (JASIK et al, “Antenna Engineering Handbook”, 1st ed., page 17-9). A vertical dipole and horizontal loop can form a rotationally polarized loop-dipole array, in which the radiating elements have a common centroid and radiation phase center. In the loop-dipole array, the magnetic and electric near fields are balanced and equal.
A more convenient form of the line-circle array is the normal mode helix (WHEELER, H. A.: “A Helical Antenna For Circular Polarization”, IRE Proc., vol. 35, December 1947, pp. 1484-1488). The normal mode helix is, so to speak, a hybrid of the inductor loaded dipole and a multiturn loop antenna.
A special form of the normal mode helix is the spherical normal mode helix antenna (SNMHA), which includes a conductive helix wound on a spherical surface. It was first described by Maxwell, as an inductor (MAXWELL, J. E.: “Electricity and Magnetism”, Oxford University Press, 3rd edition, Vol 2, 1892, pp. 304-308) and later by Wheeler as an antenna (WHEELER, H. A.: “The Spherical Coil as an Inductor, Shield, or Antenna”, IRE Proc., vol. 46, September 1958, pp. 1595-1602 & Errata, vol. 48, March 1960, p. 328)). The Maxwell Inductor—Wheeler Coil holds a special place in electromagnetics. As an antenna, it is equally magnetic and electric, circularly polarized, and electrically small. Unfortunately however, it is narrow in bandwidth.
Other types of common circularly polarized antennas include dipole turnstiles, and crossed loops. Both of which can be electrically small but narrow band.
What is needed then is a small rotationally polarized omnidirectional antenna with increased bandwidth, which may be used for high frequency (HF) applications, portable phones, and other mobile communication systems, for example. Another need is for a broadband antenna loading material that will reduce antenna size and/or a radome shell without limited bandwidth.